1. Field of the Invention
This invention relates to light sources, and in particular, it relates to light sources using solid state light emitting devices.
2. Description of the Related Art
Solid state light sources, such as light emitting diodes (LEDs), exhibit longer lifetimes, lower power consumption, manageable wavelengths and other benefits in comparison with traditional light sources such as high-pressure mercury lamps, Xenon lamps or metal halide lamps. These solid-state light sources increasingly become the alternative or even preferred choice of light sources for a variety of applications, including image projection such as rear projection TV (RPTV) or front projector, headlights or illumination lights for transportation vehicles such as automobiles, motorcycles, boats and airplanes, etc. One import requirement for a light source is to provide high brightness and high power output at the same time.
In many applications, the light emitted by the light emitting device such as LED are coupled into a waveguide or optical fiber to be transmitted to a location remote from the light emitting device. The brightness and power output of a light source are often limited by the light coupling efficiency in which light from the light emitting devices such as LEDs into the waveguide or optical fiber. In one conventional device, illustrated in FIG. 1, light from an LED chip 12 is coupled into a fiber 14 by putting the fiber end surface directly on top the LED. An LED typically emit in all directions. Using the coupling structure shown in FIG. 1, LED light emitted at an angle that is outside the numerical aperture of the fiber will be lost permanently. Thus, for example, if the numerical aperture (NA) of the fiber is about 0.5, the coupling efficiency will be about 25% (NA2).